This invention is directed to a highly stretched film having heat-sealed edges and a process and apparatus for making the film. The film has strong permanently heat-sealed edges that are nearly invisible. The thickness and integrity of the heat-sealed edges are controlled by the process of this invention, thereby improving the appearance of the wound roll.
Various cold processes for making a film with reinforced edges are known in the art. Such reinforced films have good tear characteristics to resist tearing at the edges. These films are used particularly for securing and packaging goods for shipping.
Related art provides a cold process for providing longitudinal reinforcement to a strip of film, such as in U.S. Pat. No. 4,905,451 to Jaconelli et al. Jaconelli et al. discloses runners positioned to fold zones of a strip to overlap each other to form a zone of reinforcement.
Salzsauler et al., U.S. Pat. No. 5,531,393, discloses a cold process for making a stretch film having folded edges comprising adjustable folding finger means which cause edges of a film to be folded prior to stretching and embossing the film.
Scherer, U.S. Pat. No. 5,565,222, discloses a cold process for making a stretch film having flat, double thickness hems on opposite margins of the film.
Prior art films and the processes for making films having edges have certain disadvantages. One disadvantage is that the edges are produced by cold process whereby the films are folded to form edges. The edges are easily separable during application and/or use.
Another disadvantage is that the thickness of the edges formed during cold processes is at least twice the thickness of the center portion of the film. Also, the folded edges need to be fairly wide to prevent unfolding. As a result, films having such edges are difficult to wind onto a core for storage. Unevenly wound rolls have poor appearance and cause excessive core pressure loading which results in core failure.
A process and a film produced by such process is needed to overcome the problems associated with prior art reinforced films and the cold processes to make such films.
This invention is directed to a highly stretched film having permanently heat-sealed edges that are nearly invisible. The heat-sealed edges have a width of about 0.10 inch to about 1.0 inch. The heat-sealed edges do not separate, and prevent the film from tearing.
The film edges may be folded over once (to form a two-layer folded edge) or more than once (to form folded edges having three or more layers). In order to form the edges, a film is fed to a folding apparatus before being stretched with the aid of heat. Any folding process, which folds both edges of the film, is suitable. In one process, the film advances over a first folding roll having a length which is less than the initial width of the film. As the film advances over the first folding roll, edge portions project outwardly beyond opposite ends of the first folding roll. A second folding roll is positioned relative to the first folding roll to direct the film at an angle with respect to the initial path of the film, maintaining the film under tension. As a result of said tension, the edge portions are folded downwardly through acute angles over opposite ends of the first folding roll.
A pair of folding members are positioned between the first folding roll and the second folding roll. As the film advances across the folding members, the edge portions are folded until the edge portions are in contact with a bottom surface of the film to form folded edges. This process may be repeated if folded edges having multiple (more than two) film layers are desired.
Once the folded edges are formed, the film is fed to a high speed heating and stretching apparatus comprising a bridle mechanism having a first heated roll and a second heated roll. The folded film passes through a nip upon contacting the first heated roll. The nip pressure, combined with the heat imparted by the first and second rolls, causes thermal welding of the folded edges resulting in permanent heat-sealed folds. The strength of the heat seals is maintained and, to some extent reinforced, during the hot stretching process. Because the film is folded prior to hot stretching, the hot stretch process performs dual functions of stretching the film and heat sealing the folded edges. The resulting stretched film has permanently heat-sealed edges that do not separate, and which prevent tearing of the film edges during subsequent processing and use.
With the foregoing in mind, it is a feature and advantage of this invention to provide a highly stretched film having permanently heat-sealed edges that resist tearing and separation. The heat-sealed edges are nearly invisible. The controlled, uniform thickness of the folded edges minimizes core pressure loading and improves the wound roll appearance.
It is another feature and advantage of this invention to provide an apparatus and a high speed process for forming edges on a film, heat-sealing the edges and stretching the reinforced film by about 100-450% of its initial pre-stretched length, to an ultimate length that is about 200-550% of the initial length.
It is another feature and advantage of this invention to provide an apparatus and a high speed process for heating and stretching a reinforced film having folded edges, in which the reinforced film is fed from a bridle mechanism at a rate of up to about 2,500 fpm.
The foregoing and other features and advantages of this invention will become further apparent from the following detailed description of the preferred embodiments, read in conjunction with the drawings.